The present invention relates to a process for refining noble metals from auriferous mines.
Particularly, the present invention relates to a process for extracting particles of noble metals embedded in auriferous rocks by chemical methods.
The technology for extracting noble metals currently entails the execution of a preliminary step of coarse grinding of the material that arrives from the auriferous veins and a subsequent crushing in a ball mill to obtain very fine powder.
The milled rock is then circulated in countercurrent with respect to a solution of cyanides in tanks equipped with an agitator and then in concentrating tanks to dissolve the noble metals and separate out the residual rock.
The solution is then subjected to clarification and mixed with zinc powder in order to precipitate the gold in the form of solid particles which are filtered out of the liquid mix by means of a press filter together with the excess zinc powder.
The reaction scheme is as follows:
2Au+4NaCN+1/2O2+H2O - - - 2NaAu(CN)2+2NaOH Zn+2NaAu(CN)2xe2x80x94Na2Zn(CN)4+2Au
The filter cake is then smelted in a furnace and the molten material is cast into conical ingots, on the bottom which the solid gold is separated from the slag.
The resulting crude gold ingot, which generally contains impurities such as silver, copper and other heavy metals, in addition to traces of metals of the platinum and palladium group, is first remelted in an oxidizing environment in order to slag off most of the heavy metals and is then subjected to chlorination by bubbling chlorine gas through the molten impure gold. The metals volatilize and slag off as chlorides, while the silver converts to salt silver chloride, and remains in the molten state and is separated by decantation.
Finally, the crude material containing gold, platinum and palladium is refined electrolytically with anodes consisting of crude gold ingots.
In order to separate the silver and prevent it from depositing on the anode, an AC-type current is superimposed on a DC current; the copper, palladium and platinum enter the solution as chlorides, while most of the other metals remain in the silver chloride sludge that deposits below the anode and must be treated again.
However, conventional technologies for the extraction of heavy metals are not free from drawbacks in use which lead to a loss of noble material.
A first drawback of the prior art is related to the equilibrium of the above-described zinc/auric salt reaction, which is never fully shifted to the right because unprecipitated gold ions are present in the solution and accordingly the liquid residue produced by filtration in the press filter has significant gold concentrations in view of the volumes of cyanides involved.
Another drawback of the prior art for extracting noble metals lies in the smelting steps, during which the slagging of the heavy metals inevitably also entrains noble metals, which therefore are lost. Moreover, in smelting operations there is also a loss of non-noble metals, which can have a certain economic interest when present in significant amounts.
Another drawback of the prior art is the fact that the galvanic refining operations require particular provisions for optimizing work, which however do not prevent the use of noble metal for deposition and cyanide pollution, which is difficult to manage.
The aim of the present invention is to minimize or considerably reduce the drawbacks found in the prior art.
An object of the present invention is to provide a high level recovery of noble metals in a highly pure form, thus eliminating substantially entirely the loss of noble slag from smelting and process end-fraction residues.
Another object is to provide a process for refining noble metals from auriferous mines by chemical methods with a limited production of acid fume emissions into the atmosphere.
Another object of the present invention is to provide a process for refining noble metals from auriferous mines which is simple to perform and is highly safe for workers.
Another object of the present invention is to provide a process for refining noble metals from auriferous mines which also allows non-noble metals, such as copper, zinc and cadmium, to be recovered in a pure form of considerable commercial interest.
Within the scope of this aim, these objects and others which will become apparent hereinafter, according to the present invention a process for refining noble metals from auriferous mines is provided, in which preferably the auriferous material is first milled and treated with a cyanide solution and which comprises:
advantageously a pretreatment, which comprises a step for separating the complexes of cyanide with the noble metals by means of organic ion-exchange resins and the incineration of said organic resins, preferably at a temperature which is suitable to destroy the cyanide content of the depleted resins,
a predefining step, which comprises: an etching of the material incinerated during the pretreatment with a hydrochloric-nitric solution, preferably at the boiling temperature; a subsequent step for separating a solution which contains gold, platinum, palladium, non-noble metals, in the form of chlorides, from a solid residue which contains silver chloride and end-fraction residues; the treatment of said solution with an aqueous solution of ammonium chloride; and an additional step for separating a precipitate which includes salts of platinum and palladium and a solution containing auric chloride;
specific steps for refining gold, palladium, platinum and silver.
The extracted auriferous material is advantageously subjected to a preliminary treatment which comprises the steps of grinding, cyaniding and concentration according to the operating technologies of the prior art.
In particular, the auriferous rocky material is first broken up and then ground, for example in a ball mill, in order to be reduced to a powder. The ground material is then circulated in countercurrent in a solution of cyanides, preferably inside a tank, and then transferred into one or more concentration containers.
In the pretreatment of the auriferous material, the separation step advantageously occurs in separation columns provided with resins which are capable of selectively fixing the complexes of cyanide with noble metals.
According to a first aspect of the present invention, the metals being present in the cyanide solution are flowed through one or more separation columns which include an anionic resin capable of selectively fixing the complexes produced by the reaction between the cyanide solution and the noble metals and have no affinity for non-noble metals. The fixing process is based on an ion-exchange reaction which produces a highly stable complex of noble metal trapped in an organic matrix.
The preferred resins are organic ion-exchange resins, particularly anionic resins, preferably medium-basic resins such as Relite A 470 (Mitsubishi Kasei Corporation). Resins of this kind have a pale yellow coloring and then assume an ocher yellow color when saturated with any noble metal.
When they assume this new color, they are removed, introducing new resins in the columns.
According to one embodiment, multiple separation columns connected in series are used; in this way, the first columns absorb more noble metal than the others and are depleted more rapidly. In order to ensure more uniform consumption it is possible to equip them with a plurality of bypass valves, controlled so as to allow the last regenerated column to become the last of the series, acting as an indicator column.
The liquids from which the noble metals have been extracted can be subjected to a process for recovering the non-noble metals by galvanic methods, in accordance with the prior art, to be subsequently sent to neutralization.
The depleted resins are then incinerated and preferably weighed and mixed.
Incineration of the depleted resins advantageously is carried out in a test reheat furnace operating at approximately 1000xc2x0 C. in an oxidizing atmosphere, in order to allow total destruction of the cyanide content.
Advantageously, weighing is performed in order to determine the amount of noble metals present during this step of the process.
Mixing advantageously occurs in a mixer of the static type, in order to homogenize the incinerated powder, from which the noble metal content is determined by sampling.
According to one embodiment of the invention, a sample of the mixed material is analyzed in order to determine its noble metal content (sampling).
The sample is initially mixed with flux, preferably in the ratio of 20 g of flux per 100 g of material to be smelted.
The flux advantageously includes borax, wood charcoal, sodium bisulfite, sodium nitrate and sodium metasilicate. A preferred flux is constituted by 80% borax, 10% powdered wood charcoal, 5% sodium sulfite, 2% sodium nitrate, 3% sodium metasilicate; all these percentages are by weight.
After mixing, the mixture is smelted in a furnace for example of the direct-fired type fueled by propane or methane gas, with a removable crucible. The result of the smelting is a bar which is peeled, cleaned by brushing and subjected to optional boiling in a solution of sulfuric acid in order to eliminate all traces of residual flux, then washed with demineralized water and finally dried. When the acid has fully converted into a salt, it is neutralized with hydrated lime, filtered in order to eliminate the calcium sulfate, and then discharged.
The material arriving from said incineration step is then subjected to the prerefining steps and then to the specific steps for refining the noble metals, in accordance with the process of the present invention.